The invention herein described was made in the course of or under a contract or subcontract thereunder, (or grant) with the Department of the Air Force.
This invention relates to a digital linear-phaseshift convolutional filter for filtering a series of binary number electrical signals, each binary number electrical signal representing the amplitude of a sample of an analog electrical signal sampled at a predetermined rate.
Generally, all communications systems of electronic character require filtering of the electrical waveforms present in the communication system. Except in those situations wherein it is desired to remove previously introduced distortions of a signal with a non-linear phase filter equalizer, it is generally desired to provide an ideal filter in which the input electrical signal, over the passband of interest, appears at the output of the filter with an amplitude which is constant and with a phase shift which is a linear function of the frequency content of the input electrical signal. In practice, a filter which does not attenuate the input signal to produce amplitude distortion is very difficult, if not impossible, to obtain. However, a filter which has linear phase shift and desirable sharp cutoff at the upper and lower frequency limits of the passband can be obtained.
The ideal filter described in the preceding paragraph can be closely approximated in a digital convolutional filter. With a filter of this kind, an analog electrical signal is sampled at a predetermined rate to generate binary number electrical signals representing the amplitudes of the samples of the analog electrical signal. In a manner analogous to a transversal filter utilized in analog filtering systems, the binary number electrical signals representative of the sample amplitudes are multiplied by weight numbers stored in a memory, these weight numbers being analogous to the tap weights of a transversal filter in an analog system. For each sample presented at the input of the digital filter, an output binary number electrical signal is generated. This generated output binary number electrical signal has an amplitude determined by the summation of the products produced by multiplying previous sample values by the weight numbers stored in the memory.
In a digital filter of the kind described above, the filtering operation is performed utilizing a predetermined number of the input binary number electrical signals representing samples of the analog electrical signal. Usually, these binary number sample values are stored in a shift register which is updated by the introduction of a new sample as it appears at the filter input. When the new sample value is introduced into the shift register, the oldest of the stored sample valve is discarded. In a linear-phase-shift digital filter, the weight numbers stored in the memory necessarily are symmetrical about a central value, where an odd number of weight numbers is utilized. Where an even number of weight numbers is utilized, they are symmetrical in the sense that for each weight number there is a second and corresponding weight number which is utilized by the digital filter. The symmetry of the weight numbers may be even or odd; for even symmetry, for every weight number h =F (A), there exists a corresponding weight number h=f(-A). For odd symmetry, for every weight number h=f(-A), there exists a corresponding weight number -h = f(-A).
The present invention utilizes the symmetry of the weight numbers in a digital linear-phase-shift convolutional filter to reduce the number of binary number electrical signal multiplications required by prior art convolutional filters, thereby, either to reduce the number of multipliers required in the electronic circuitry or to reduce the required computation time. Moreover, the invention reduces the electronic hardware requirements and lowers the electronic system power requirements. Furthermore, the number of filter weight numbers required to be stored in the filter memory may be reduced or the number of logic control elements required to select the weight numbers may be reduced.
The following U.S. Pat. Nos. are illustrative of the prior art: 2,980,871 to Cox; 3,315,171 to Becker; 3,639,848 to Elliott; and 3,717,756 to Stitt.